1. Field of the Invention
The present invention relates to switching mode electronic drive circuits for inductive loads and, in particular, to monolithically integrated circuits of this type.
2. Description of the Prior Art
When driving inductive loads, e.g. electric motors, in a switching mode, the necessity of providing suitable circuit means for recirculating the discharge current of the energy stored in the inductance following each current interruption is well known. This in order to limit the overvoltage (negative in the case of an inductive load driven from the high side of the supply and therefore with a terminal connected to ground) across the load due to the discharge of the energy stored in the inductance during the preceding charging stage, i.e. at the beginning of the current flow through the load. These negative voltage peaks (below ground) if not properly controlled in intensity, may cause the breakdown of junctions and, in case of integrated circuits, the triggering of parasitic transistors and of problems deriving therefrom.
There are many peculiar applications wherein the inductive load, e.g. an electric motor, must be frequently stopped in order to change speed of rotation, commonly by having another cooperating motor taking over. In a situation of this type it is necessary to provide means for a "slow" recirculation of the discharge current during the switching mode driving of the motor, as well as means, selectable through a switch whenever the action of the motor must be completely interrupted, capable of permitting a "fast" recirculation of the discharge current of the load. Essentially the slow recirculation means discharge the inductance at a relatively limited negative voltage (below ground voltage) while the fast recirculation means permit the discharge of the energy stored in the inductance at a relatively higher negative voltage (below ground voltage) for speeding up the discharge process.
In these instances, the commonly used circuit according to the prior art is depicted in FIG. 1. The load L driving transistor (high side driver) is the transistor Td, which is base-driven by a driving signal Vin. A fast recirculation of the inductance L discharge current, when the current flowing through the load must estinguish quickly, occurs through the recirculation path formed by the zener diode Dz and by the diode D. When a slow recirculation is desired, e.g. for maintaining a certain "continuity" of current flow through the load, the zener diode Dz is shortcircuited by means of a switch S driven by an arresting signal of the electrical supply of the inductive load L. This known solution requires three power devices, namely the diode D and the zener Dz, both capable of recirculating the discharge current of the inductive load, as well as a switch S also capable of withstanding the same recirculation current. This implies a large area requirement.